Add client filtering of conversation part types for authorized users

# Problem

Add the ability for authorized users (admin) to filter by conversation part type.

This is currently blocked by 1638 and 1689.

# Add client-filtering of parts based on authorizaton
Wrap non-comment parts in the conversation thread under a `{{#if canAdminister project}}` helper, to hide non-comment parts from users who aren't admins on project.

## Add integration tests to support this change
Render a thread with one of each supported part type, register a fake project ability with `canAdminister: true`
Render a thread with one of each supported part type ,register a fake project ability with `canAdminister: false`, see if just the comment part is rendered
See `tests/integration/components/task-assignment-test.js` on how to register a fake ability

## References #1636 #1638, #1689

Progress on: #1597

先查看阻塞项 #1638 和 #1689，然后以 tests/integration/components/task-assignment-test.js 作为注册 fake ability 的参考。为包含每种受支持 part type 的 thread 添加集成覆盖，并分别测试 canAdminister 为 true 和 false 的情况；完成标准是 admins 能看到所有 parts，而 non-admins 只能看到 comments。

由索引模型根据 Issue 内容生成。